Answer:because no matter what x is, the equation on both sides are the same
Step-by-step explanation: for example we will replace x with 5
7(5) + 3=7(5) + 3
7 times 5 equals 15, plus 3 equals 18, now lets look at the other side, 7 times 5 equals 15, plus 3 equals 18, there fore u could use the number 777,777,777 and you would get the same answer on both sides